pg_restore: from TOC entry 4295; 1255 596569951 FUNCTION
gist_stratnum_btree(integer) postgres
pg_restore: error: could not execute query: ERROR: could not find
function "gist_stratnum_btree" in file "/usr/local/pgsql/lib/btree_gist.so"
Command was: CREATE FUNCTION "public"."gist_stratnum_btree"(integer)
RETURNS smallint
LANGUAGE "c" IMMUTABLE STRICT PARALLEL SAFE
AS '$libdir/btree_gist', 'gist_stratnum_btree';
The solution was to somehow restart 18beta1, drop btree_gist and all its
dependent constraints / indexes / objects , pg_upgrade and finally
re-create extension and dependent objects.